United Nations Secretary-General Antonio Guterres says this week's inaugural meeting of a committee tasked with drafting a new Syrian constitution presents a "unique opportunity" for peace.
The U.N. chief said Tuesday that he expects the 150-member committee to work "in good faith."
Representatives from the Syrian government, opposition and civil society are to begin meeting Wednesday in Geneva.
Repeated U.N. efforts to host talks on ending Syria's eight-year civil war have largely failed.
Guterres stressed that "meaningful engagement" by committee members must be accompanied by a cessation of hostilities across the country. He says that would facilitate "a broader political process."
Syrian President Bashar al-Assad's government has all but won the war militarily with the help of Russia. Syria's last rebel stronghold is in the northwestern province of Idlib.